The present invention pertains to write drivers which operate the write heads in mass data storage systems, particularly magnetic data storage systems.
In magnetic data storage systems, a magnetic write head writes, or records, data as a sequence of ones and zeros on a moving magnetic medium, such as a magnetic tape or disc. The write head uses an inductive coil to generate magnetic fields which magnetize portions of the medium and thus form patterns representing the ones and zeros on the medium. A "one" is written as a transition in the magnetic pattern on the medium by changing the direction of current through the inductive coil. Thus, writing specific data requires selectively changing the direction of current through the coil of the write head.
Changing the direction of current through the coil is the function of a write driver. A typical write driver includes an H-switch drive circuit and a control circuit. The H-switch drive circuit, which resembles an "H," has two head pins connected to a write head to form the cross-bar of the H, and four drive transistors, which form the upper and lower legs of the H. More specifically, the two drive transistors that form the left upper and lower legs of the H are connected respectively between the left head pin and respective positive and negative supply voltages. And, the two that form the right upper and lower legs of the H are connected respectively between the right head pin and the respective positive and negative supply voltages.
The control circuit, which responds to data signals, selectively operates the four drive transistors as on-off switches, thereby controlling current direction through the write head. Specifically, to direct current left to right through the write head, the control circuit activates (or turns on) the left-upper and the right-lower drive transistors and turns off the right-upper and the left-lower drive transistors. Conversely, to direct current right to left through the write head, the control circuit turns off the left-upper and right-lower drive transistors and turns on the right-upper and the left-lower drive transistors.
Write drivers typically suffer from two problems: the first concerns limitations to switching rate, and the second concerns glitching. Switching rate, a measure of how fast the write driver reverses current direction, defines the spatial transitions written on a magnetic medium, with higher switching rates requiring sharper, more distinctive transitions than lower switching rates. Ultimately, a higher switching rate yields closer data spacing and thus greater data capacity for a magnetic medium.
One factor limiting switching rate is the inherent switching limitation of the drive transistors comprising the H-switch drive circuit. The drive transistors have inherent structural, or parasitic, capacitances which require charging or discharging during switching and thus prevent the drive transistors from instantaneously switching on or off. This charging and discharging limits the switching rate of the write driver.
The second problem, glitching, occurs during reversals of write current direction. Specifically, during reversals of write current direction, the write head exhibits self-inductance, a phenomenon which produces a voltage spike at one of the head pins (depending on the desired current direction). The voltage spike not only shoots several volts above the positive supply voltage of the write driver, but also incites a ringing, or oscillating, voltage that last several nanoseconds before eventually decaying to a negligible level.
The voltage spike and related ringing are troublesome because they feedback through the capacitances of the upper left and right drive transistors to the portion of the control circuit that sets the voltages at the control nodes of these transistors. This feedback causes spurious increases or decreases in the voltages at these nodes, not only disrupting operation of the upper drive transistors, but ultimately producing glitches, or irregularities, in the magnetic patterns written by the write head.
The prior art has addressed the switching-rate limitation and glitching problems with limited success. For example, U.S. Pat. No. 5,296,975 to Contreras discloses a write driver having two MOSFET (metal-oxide-semiconductor field effect transistor) inverters to charge and discharge the capacitances of the upper drive transistors. Contreras also discloses a damping resistor coupled in parallel with its write head to hasten the decay of the ringing voltage. But, Contreras' write drive still suffers from at least four drawbacks.
First, the two MOSFET inverters are large devices and thus occupy significant area of an integrated-circuit implementation. Second, the damping resistor increases power usage without appreciably reducing glitching. Third, Contreras uses a dual current-mirror control circuit which is not only complex but also costly in terms of integrated-circuit area. Fourth, Contreras' write driver uses both bipolar junction transistors and MOSFETs which makes it more complicated to manufacture than a pure bipolar or MOS design.
Thus, there is a need for a simple, more-easily manufactured write driver having circuitry for not only increasing switching rate of the drive transistors, but also isolating the control circuit from the write head to reduce glitching.